Double casting method and apparatus

ABSTRACT

The present invention relates to a duplex casting apparatus and a duplex casting method. The duplex casting apparatus includes a casting chamber in which a space portion is formed; an injection tube mounted below the casting chamber and into which a molten metal is injected; a mold detachably connected to the injection tube and having a cavity; a filling medium filled between an inner portion of the casting chamber and the mold; and a pressure regulation means disposed above the casting chamber and decompressing the inner portion of the casting chamber.

TECHNICAL FIELD

The present invention relates to a method and apparatus for duplex casting. More particularly, the present invention relates to a method and apparatus for duplex casting which apply decompression casting and gravity casting together to a mold, such that gas pores and bubbles in a product are removed to prevent a casting defect, and a non-fill problem is resolved to easily produce high heat resistant cast steel products.

BACKGROUND ART

A generally-used metal mold casting method is one of the most fundamental methods of metal molding, and is used to mass-produce products of the same form. In the casting method, scrap, pig iron, alloy iron, or non-ferrous metal raw materials are injected into a furnace, heated and melted, then poured into a mold formed of sand or a metallic material, and cooled and coagulated to manufacture products. The principle is that, after a molten metal is injected into a mold manufactured in a desired shape and formed, a metallic object of the same shape is formed when the molten metal is coagulated. Although casting enables products to be mass-produced when a mold design is complete, a defect in the casting in which an unwanted non-metallic component (inclusion) is included in a final product instead of a metallic substance frequently occurs in a coagulation process of a cast. In addition, a molten metal injected by being melted in the atmosphere may melt and absorb a considerable amount of hydrogen gas, oxygen gas, nitrogen gas, and the like., decreasing solubility during coagulation and generating bubbles, and a partial gas which is not released may remain and cause surface defects.

A method of casting using a vacuum technology to prevent such defects is called a vacuum (decompression) casting method. Vacuum casting may be used by performing melting in the atmosphere and casting under a vacuum condition, or processing a molten metal melted in the atmosphere in a vacuum state when receiving the molten metal in a ladle and casting under the vacuum condition or in the atmosphere.

When the vacuum casting method is used, harmful gas components may be easily removed and mechanical properties of products may become excellent. Thus, this casting method is used in manufacturing products such as stainless steel, heat resistant steel, tool steel, bearing steel, magnetic materials and the like.

A countergravity vacuum assisted casting method, which is one type of the vacuum casting method, was developed in the 1970s and 1980s by Hitchiner Manufacturing Co. in the U.S. etc., and is a method of rapidly suctioning a molten metal into a mold through a vacuum system by inserting a ceramic tube into the molten metal. Korean Registration Patent Nos. 0947948 and 0801815 are representative patents related to countergravity vacuum assisted casting and vacuum casting apparatuses. However, since the casting method of Hitchiner Manufacturing Co. which is described in the registered patents is limited to precision casting product production by a ceramic mold, the casting method has a serious problem of causing many casting defects such as mixing of inclusions which occur in product production by casting using a sand mold since the sand mold cannot withstand a molten metal being rapidly suctioned thereinto. In addition, this technology is a method of producing a product by suctioning a molten metal into a mold using only a vacuum pressure, such that not only small pores but also large holes such as worm holes are formed in a product in some cases due to a rapid flow of molten metal, and many non-fill casting defects are on partial surfaces of a product.

Particularly, since a molten metal has to be rapidly filled into a ceramic cell mold using a vacuum pump in Korean Registration Patent No. 0947948, a severe problem of mold damage may occur with a low-strength sand mold, and when a molten metal is filled in a cavity of a mold and a case is rotated at 300 RPM to 600 RPM, dimensions of a product may change due to the molten metal damaging or deforming the mold, and may lead to severe defects in quality, such that a high-price ceramic cell mold has to be used, which is disadvantageous in terms of original cost. Furthermore, since the ceramic cell mold for precision casting does not naturally collapse after the molten metal is solidified, the ceramic cell mold cannot be applied to inner inserted type products such as a turbine housing.

In addition, the ceramic cell mold is used at temperatures of 900° C. or higher. Here, since a long time of 600 seconds or more is required until the molten metal is coagulated due to the high temperature of the mold, the ceramic cell mold has a severe weakness in terms of productivity. If the mold is moved within a shorter time, not only the molten metal in a vertical riser of Korean Registration Patent No. 0947948, but also uncoagulated molten metal in a product connected to a gate may exit from a source and lead to severe quality problems. Thus, as an improvement method, after filling the molten metal into a cavity of a mold, centrifugal force is used to rotate the case until the product is completely solidified in order to attempt an improvement in productivity. However, a long time of 300 seconds or more is still required until the product is completely solidified.

In addition, since the case is rotated in Korean Registration Patent No. 0947948, arranging the product mold in a forward direction in which the product mold becomes horizontally symmetrical is effective, which causes limitations in a shape of the product. In addition, since the ceramic cell mold has to be positioned at a central portion of the case for the centrifugal force to be uniformly applied throughout the product, a single ceramic cell mold to which many mold cavities are radially attached is used.

In addition, in the mold for precision casting, since an injection tube was expendably used once, and the injection tube was manufactured of the same material as the ceramic cell mold and was connected to the ceramic cell mold, the mold was not separated even when the molten metal was solidified, and thus could not be reused.

Due to these problems, the countergravity vacuum assisted casting technology of Hitchiner Manufacturing Co. was not commercialized in sand mold casting products, and no company around the world has succeeded in developing a countergravity vacuum assisted casting technology using a suction tube applied to the sand mold casting products so far.

Therefore, the present inventors have repeated studies on casting methods for overcoming the above problems, and produced a high-temperature heat resistant steel stainless steel product by applying a duplex casting technology which removes a vacuum pressure in a sand mold, instead of ceramic precision casting mold, to refill a molten metal which is suctioned but not yet coagulated by gravity, thus completing the present invention.

DISCLOSURE Technical Problem

An aspect of the present invention is to provide a duplex casting method which is capable of preventing shrinkage defects, bubble defects, and other casting defects when casting a product.

Another aspect of the present invention is to provide a duplex casting method through which inclusions included in a molten metal may be easily removed.

Still another aspect of the present invention is to provide a duplex casting method in which a degree of precision is excellent, and manufacturing a product of a complex shape is possible.

Yet another aspect of the present invention is to provide a duplex casting method capable of saving on costs and improving productivity.

Yet another aspect of the present invention is to provide a duplex casting apparatus.

Technical Solution

One aspect of the present invention relates to a duplex casting apparatus. The duplex casting apparatus includes a casting chamber in which a space portion is formed; an injection tube mounted below the casting chamber and into which a molten metal is injected; a mold detachably connected to the injection tube and having a cavity; a filling medium filled between an inner portion of the casting chamber and the mold; and a pressure regulating means disposed above the casting chamber and decompressing the inner portion of the casting chamber.

In one embodiment, a riser accommodating the molten metal by coming in communication with an upper portion of the cavity in the mold and discharging the molten metal to the cavity may be further included.

In one embodiment, the riser may be formed in a size of about 30 volume % to about 120 volume % of the mold.

In one embodiment, a melting furnace for melting a raw material to form a molten metal may be further included.

In one embodiment, an inner diameter of the injection tube may be larger than an inner diameter of an inlet of the cavity.

In one embodiment, a ratio of the inner diameter of the cavity to the inner diameter of the injection tube may be about 1:1.1 to about 1:3.

In one embodiment, the pressure regulating means may include a suction screen, a vacuum pump, and a suction tube.

Another aspect of the present invention relates to a duplex casting method using the duplex casting apparatus. The duplex casting method includes decompressing the casting chamber into which the filling medium is filled by the pressure regulation means and performing first injection of a molten metal into the cavity inside the mold along the injection tube by the pressure difference with the outside; and undoing the decompression of the casting chamber before the molten metal of the first injection into the cavity is coagulated and performing second injection of the molten metal by gravity.

In one embodiment, the molten metal may also be injected into the riser formed to come in communication with the upper portion of the cavity in the mold during the first injection, and the molten metal injected into the riser may be discharged by gravity and the second injection into the cavity may be performed when the decompression of the casting chamber is undone.

In one embodiment, the method may include inserting the mold into the casting chamber on which the injection tube is mounted to be coupled to the injection tube, and performing the first injection of the molten metal by decompression after filling the inner portion of the casting chamber with the filling medium.

In one embodiment, a decompression rate of the casting chamber may be regulated to regulate a speed at which the molten metal is injected into the mold.

The injection tube, the pressure regulation means, and the filling medium may be reused.

Advantageous Effects

When a method and apparatus for duplex casting of the present invention is applied, shrinkage defects and bubble defects may be prevented, inclusions included in a molten metal may be easily removed to prevent casting defects, a degree of precision may be excellent, manufacturing a product of a complex shape may be possible, and reusing an inlet, a pressure regulation means, a filling medium, and the like may be possible, thereby enabling costs to be saved and productivity to be improved.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a cross-section of a duplex casting apparatus according to one embodiment of the present invention.

FIG. 2(a) illustrates a cross-section of an injection tube according to one embodiment of the present invention, and FIG. 2(b) illustrates a cross-section of an injection tube according to another embodiment of the present invention.

FIG. 3 illustrates a duplex casting apparatus according to another embodiment of the present invention.

MODES OF THE INVENTION

An aspect of the present invention relates to a duplex casting apparatus.

FIG. 1 illustrates a cross-section of a duplex casting apparatus according to one embodiment of the present invention. Referring to FIG. 1, a duplex casting apparatus 100 includes a casting chamber 10 in which a space portion is formed; an injection tube 20 mounted below the casting chamber 10 and into which a molten metal is injected; a mold 30 detachably connected to the injection tube 20 and having a cavity 35; a filling medium 40 filled between an inner portion of the casting chamber 10 and the mold 30; and a pressure regulation means 60 disposed above the casting chamber 10 and decompressing the inner portion of the casting chamber 10.

A space portion may be formed in the casting chamber 10 to enable the filling medium 40 to be filled and fix the mold 30, and the casting chamber 10 may be decompressed by the pressure regulation means 60 to a pressure equal to or less than atmospheric pressure to provide a decompressed atmosphere in the mold 30.

The injection tube 20 is dipped into the casting chamber 10, mounted below the casting chamber 10, and fixed, and a portion of the injection tube 20 protrudes and is dipped in the molten metal to inject the molten metal into the mold 30. Although one injection tube 20 is illustrated in FIG. 1, a plurality of injection tubes 20 may also be installed at the casting chamber 10 in other embodiments.

The injection tube 20 formed of a common material may be used. For example, the injection tube 20 may be formed of a fireproof material such as a ceramic that may prevent a reaction when coming in contact with the molten metal, but the material is not limited thereto.

Referring to FIG. 1, an inner diameter d1 of the injection tube 20 may be larger than an inner diameter d2 of the cavity 35 in the present invention. In this case, a time for which the molten metal stays may be increased even when the decompression is undone. In one embodiment, a ratio of the inner diameter d2 of the cavity 35 to the inner diameter d1 of the injection tube 20 (d2:d1) may be about 1:1.1 to about 1:3. Preferably, the ratio may be about 1:1.5 to about 1:2.5. More preferably, the ratio may be about 1:2. In the above ranges, the time for which the molten metal stays may be increased even when the decompression of the casting chamber 10 is undone.

A shape of an inlet of the injection tube 20 may be designed to be fitted to the mold. FIG. 2(a) illustrates a cross-section of the injection tube 20 according to one embodiment of the present invention, and FIG. 2(b) illustrates a cross-section of an injection tube 21 according to another embodiment of the present invention. As illustrated in FIG. 2(a) and FIG. 2(b), a portion coming in contact with a floor surface of the casting chamber may be wide and gradually narrowed upward.

The mold 30 is detachably connected to the injection tube 20, and may have the cavity 35 of a desired product shape.

The mold 30 formed of a common material may be used. For example, a sand mold may be used as the mold 30. In one embodiment, the mold 30 may be used after being manufactured by obtaining mixed sand which includes sand, a water-soluble phenolic resin, and an ester based curing agent, and inserting the mixed sand into a mold, but the material is not limited thereto.

Common sand may be used as the sand. For example, silica sand, zircon sand, chromite sand, olivine sand, or alumina sand may be used. In one embodiment, the sand having a diameter of about 0.05 mm to about 1 mm may be used.

In one embodiment of the present invention, a plurality of molds 30 may be installed in the casting chamber 10. For example, one to nine molds 30 may be installed, such that the cost may be saved and the productivity may be improved through efficient space utilization.

As a resin attached around the sand disappears due to heat in the sand mold after a predetermined amount of time, the sand mold naturally collapses. Consequently, the cost saving effect is improved because the mold 30 and the ceramic injection tube 20 are naturally separated such that the injection tube 20 attached to the case is reusable, and the productivity improving effect is improved because the injection tube 20 is fixed to the casting chamber 10 as illustrated in FIG. 1 and the mold 30 may be continuously replaced by itself.

The filling medium 40 may be filled in the casting chamber 10 and around the mold 30 to support the mold 30. The filling medium 40 formed of a common material may be used. For example, fireproof particles such as mixed sand or sand may be used, but the material is not limited thereto.

In one embodiment, the filling medium 40 may be filled while applying vibration to the casting chamber 10 for a predetermined amount of time, thereby enabling the filling medium 40 to be uniformly filled while increasing the filling density.

In one embodiment, the duplex casting apparatus 100 may further include a riser 50 accommodating the molten metal by coming in communication with an upper portion of the cavity 35 in the mold 30 and discharging the molten metal to the cavity by gravity. The riser 50 may be included to prevent inclusions from being included in a product or casting defects such as bubbles when the mold 30 is coagulated. Referring to FIG. 1, as in one embodiment of the present invention, the riser 50 may be disposed above the cavity 35. In one embodiment, a bottom surface of the riser 50 may be disposed above a top surface of the cavity 35. In one embodiment, an interval between the bottom surface of the riser 50 and the top surface of the cavity 35 may be about 1 cm to about 100 cm.

Consequently, when the molten metal is filled from a lower portion of the mold 30 and moves toward the riser 50, inclusions and impurities remaining in the initial unstable molten metal may move upward to be removed from the product, and the molten metal injected into the riser 50 is injected into the cavity 30 below the riser 50 from the riser 50 disposed thereabove by gravity, thereby enabling bubbles and air included in the molten metal in the cavity 35 to be removed, preventing shrinkage defects and a bubble defects of a product, enabling the inclusions included in the molten metal to be easily removed to prevent casting defects, enabling an excellent degree of precision, and enabling a product of a complex shape to be produced.

In one embodiment, the riser 50 may be formed in a size of about 30 volume % to about 120 volume % of the mold 30. In one embodiment, the riser 50 may be formed in a size of about 50 volume % to about 80 volume % of the mold 30.

Inclusions included in the molten metal may be easily removed in the above ranges, and the bubbles and the air remaining in the mold 30 may be easily removed.

The pressure regulation means 60 may be disposed above the casting chamber 10, and may regulate a pressure in the casting chamber 10 to decompress the casting chamber 10. In one embodiment, the pressure regulation means 60 may include a suction screen 64, a vacuum pump (not shown), and a suction tube 62. Referring to FIG. 1, the suction screen 64 is connected to an upper portion of the casting chamber 10, the suction tube 62 is connected to one surface of the suction screen 64, and the suction tube 62 may come in contact with the vacuum pump (not shown) to regulate the pressure in the casting chamber 10 by operating the vacuum pump.

When the vacuum pump included in the pressure regulation means 60 is operated, the suction screen 64 may play a role of passing only gas through the suction tube 62 and preventing suction of the filling medium 40.

The vacuum pump (not shown) may regulate the pressure of the casting chamber 10 to remove gas inside the mold 30 and regulate a speed at which the molten metal is injected.

A common vacuum pump may be used as the vacuum pump. For example, an ionic pump, a diffusionic pump, and the like may be used, but the type of the pump is not limited thereto.

When the vacuum pump operates, the suction tube 62 may discharge gas in the casting chamber 10 to decompress the pressure in the casting chamber 10. Here, a speed of decompressing the inner pressure of the casting chamber 10 may be regulated to regulate a speed at which the molten metal is injected.

FIG. 3 illustrates a duplex casting apparatus 100 according to another embodiment of the present invention. Referring to FIG. 3, the duplex casting apparatus 100 may further include a melting furnace 70 for melting a raw material to form a molten metal. Referring to FIG. 3, in one embodiment, the melting furnace 70 may be disposed at a lower portion of the duplex casting apparatus 100 and the injection tube 20 of the duplex casting apparatus 100 may be dipped in the molten metal in the melting furnace 70 to inject the molten metal by the decompression of the casting chamber 10. In one embodiment, the injection tube 20 may be dipped in the molten metal formed in the melting furnace 70 in a depth of about 3 cm to about 10 cm. Under the above condition, outside air is maximally prevented from being introduced into the molten metal to prevent casting defects, and the speed at which the molten metal is injected may be regulated.

In one embodiment, a common furnace may be used as the melting furnace 70. For example, an electric furnace including a coil type heating unit such as tungsten and Kanthal may be used to heat a raw material accommodated therein and form the molten metal.

Another aspect of the present invention relates to a duplex casting method using the duplex casting apparatus 100. In one embodiment, the duplex casting method may include decompressing the casting chamber 10 to perform first injection of the molten metal into the cavity 35; and undoing the decompression to perform second injection of the molten metal.

More specifically, the duplex casting method may include decompressing the casting chamber 10 into which the filling medium 40 is filled by the pressure regulation means 60 and performing the first injection of the molten metal into the cavity 35 inside the mold 30 along the injection tube 20 by the pressure difference with the outside; and undoing the decompression of the casting chamber 10 before the molten metal of the first injection into the cavity 35 is coagulated, performing the second injection of the molten metal by gravity, and disassembling the casting chamber to separate the mold.

In one embodiment, the duplex casting method may include inserting the mold 30 into the casting chamber 10 on which the injection tube 20 is mounted to be coupled to the injection tube 20, and performing the first injection of the molten metal by decompression through the pressure regulation means after filling the inner portion of the casting chamber 10 with the filling medium 40.

Hereinafter, each step of the duplex casting method will be described in detail.

(a) Forming a Molten Metal

This step is a step of melting a raw material accommodated in the melting furnace 70 to form a molten metal. An electric furnace may be used as the melting furnace 70, and a heating coil included in the electric furnace may be used to heat and melt the raw material, but embodiments are not limited thereto.

(b) First Injection of the Molten Metal

This step is a step of filling the filling medium 40 between the casting chamber 10 and the mold 30, then decompressing the inner pressure of the casting chamber 10 through the pressure regulation means 60. In one embodiment, the decompressed pressure may be regulated to about 10⁻² torr to about 10⁻⁵ torr. Under the above condition, a speed at which the molten metal is injected may be easily regulated, and casting defects may be prevented.

After dipping the injection tube 20 of the decompressed casting chamber 10 in the manufactured molten metal, the first injection of the molten metal into the cavity 35 is performed in the mold 30 along the injection tube 20 by a pressure difference between the casting chamber 10 and the outside.

In one embodiment, the injection tube 20 may be dipped in the molten metal formed in the melting furnace 70 in a depth of about 3 cm to about 10 cm, and the molten metal may be injected. Under the above condition, outside air is maximally prevented from being introduced into the molten metal to prevent casting defects, and the speed at which the molten metal is injected may be easily regulated.

In one embodiment, during the decompression, a decompression rate of the casting chamber 10 may be regulated to regulate a speed in which the molten metal is injected into the mold 30. Specifically, in order to minimize turbulence in accordance with a desired product shape according to data obtained through repeated product manufacture, the decompression rate applied by the pressure regulation means 60 may be regulated to regulate the speed at which the molten metal is injected. For example, the decompression rate may be regulated to about 0.1 cm/s to about 30 cm/s. When the injection speed is rated as above, casting defects such as a non-fill problem which occurs when bubbles are generated by a turbulence phenomenon of the molten metal and filling of the molten metal in a fine portion of the mold 30 is disturbed may be prevented.

In another embodiment, during the decompression, the decompresson rate the casting chamber 10 may be regulated to also regulate a speed at which the molten metal is injected into the riser 50 formed to come in communication with the upper portion of the cavity 35 during the first injection.

(c) Second Injection of the Molten Metal

This step is a step of undoing the decompression of the casting chamber 10 before the molten metal of the first injection into the cavity 35 is coagulated to perform the second injection of the molten metal by gravity. The first injection of the molten metal is a motion from bottom to top, while the second injection of the molten metal is a motion from top to bottom.

In one embodiment, the molten metal of the first injection into the cavity 35 begins directional coagulation from an outside of the cavity 35. Consequently, inclusions (or impurities) with a relatively low density may be coagulated on a surface of the cavity 35 and easily removed. In addition, pores and air bubbles formed at the cavity 35 during the second injection of the molten metal may be easily removed.

In another embodiment, when the riser 50 coming in communication with the upper portion of the cavity 35 in the mold 30 is formed, the molten metal injected into the riser 50 may be discharged by gravity when the decompression of the casting chamber 10 is undone and the second injection into the cavity 35 may be performed. The pores and bubbles formed at the cavity 35 may be easily removed when the second injection into the mold 30 is performed on the molten metal injected into the riser 50 by gravity.

(d) Disassembling

This step is a step of disassembling the casting chamber 10 after the second injection of the molten metal and obtaining a product formed in the mold 30.

FIG. 3 illustrates the duplex casting apparatus 100 according to another embodiment of the present invention. Referring to FIG. 3, the injection tube 20, the pressure regulation means 60, and the filling medium 40 are easily detachable from a casting chamber 10 a. For example, after manufacture of a desired product using the duplex casting method is complete, the casting chamber 10 a may be separated and disassembled, and the injection tube 20, the pressure regulation means 60, and the filling medium 40 may be mounted on another casting chamber 10 b and reused, thereby having a cost saving effect.

Hereinafter, configurations and actions of the present invention will be described in more detail using preferred embodiments of the present invention. However, the embodiments are merely provided for illustrative purposes and cannot be construed as limiting the present invention.

Since contents which are not described here may be easily inferred by those of ordinary skill in the art, the description thereof will be omitted.

EMBODIMENT

The injection tube 20 formed of a ceramic material which had a shape illustrated in FIG. 2(a) and was 200 mm long was installed at a bottom portion of the casting chamber 10 of a cylindrical shape which had a diameter of 600 mm and a height of 800 mm.

Next, the mold 30 having the cavity 35 was manufactured using mixed sand manufactured to include sand, water-soluble phenolic resin, and an ester based curing agent, and the mold 30 was overlaid on an inlet of the injection tube 20 and disposed in the casting chamber 10. Here, the injection tube with an inner diameter d1 of Φ50 mm and the cavity with an inner diameter d2 of an inlet of Φ25 mm were used. The riser 50 coming in communication with an upper portion of the cavity 35 was mounted on the mold 30. Here, the riser 50 was formed to have a size of 65 volume % of an overall volume of the mold 30.

Here, the mold 30 formed as illustrated in FIG. 1 was overlaid on the injection tube 20 to prevent a gap from being formed. The filling medium 40 was filled between the casting chamber 10 and the mold 30 while vibration was applied thereto.

The melting furnace (electric furnace) 70 which included a tungsten coil type heating unit was prepared, and a raw material was inserted thereinto and heated to manufacture a molten metal. Then, the pressure regulation means 60 which included the suction screen 60, the suction tube 62, and the vacuum pump (not shown) was mounted on the casting chamber 10, and the vacuum pump (not shown) was operated to maintain the inner portion of the casting chamber 10 in a vacuum state of 10⁻³ torr. Then, the ceramic injection tube 20 was dipped in the molten metal manufactured in the electric furnace at a depth of 50 mm, and the first injection of the molten metal into the injection tube 20 was performed to inject the molten metal up to the mold 30 and the riser 50. The vacuum pump was turned off after 90 seconds, and before the molten metal of the first injection was coagulated, the second injection of the molten metal injected up to the riser 50 into the cavity 35 was performed by gravity. The casting chamber 10 was moved after 120 seconds to be positioned on a disassembling conveyor, and a product formed in the cavity 35 in the mold 30 was obtained. Here, casting defects such as bubbles, worm holes, non-filled portions etc. were not found in the obtained product. 

1. A duplex casting apparatus comprising: a casting chamber in which a space portion is formed; an injection tube mounted below the casting chamber and into which a molten metal is injected; a mold detachably connected to the injection tube and having a cavity; a filling medium filled between an inner portion of the casting chamber and the mold; and a pressure regulation means disposed above the casting chamber and configured to decompress the inner portion of the casting chamber.
 2. The duplex casting apparatus according to claim 1, further comprising a riser configured to accommodate the molten metal by coming in communication with an upper portion of the cavity in the mold and discharge the molten metal to the cavity.
 3. The duplex casting apparatus according to claim 1, wherein the riser is formed in a size of about 30 volume % to about 120 volume % of the mold.
 4. The duplex casting apparatus according to claim 1, further comprising a melting furnace configured to melt a raw material to form a molten metal.
 5. The duplex casting apparatus according to claim 1, wherein an inner diameter of the injection tube is larger than an inner diameter of an inlet of the cavity.
 6. The duplex casting apparatus according to claim 5, wherein a ratio of the inner diameter of the cavity to the inner diameter of the injection tube is about 1:1.1 to about 1:3.
 7. The duplex casting apparatus according to claim 1, wherein the pressure regulation means comprises a suction screen, a vacuum pump, and a suction tube.
 8. A duplex casting method comprising: decompressing the casting chamber into which the filling medium is filled by the pressure regulation means and performing first injection of a molten metal into the cavity inside the mold along the injection tube by the pressure difference with the outside; and undoing the decompression of the casting chamber before the molten metal of the first injection into the cavity is coagulated and performing second injection of the molten metal by gravity.
 9. The duplex casting method according to claim 8, wherein: the molten metal is also injected into the riser formed to come in communication with the upper portion of the cavity in the mold in the first injection; and the molten metal injected into the riser is discharged by gravity and the second injection into the cavity is performed when the decompression of the casting chamber is undone.
 10. The duplex casting method according to claim 8, further comprising inserting the mold into the casting chamber on which the injection tube is mounted to be coupled to the injection tube, and performing the first injection of the molten metal by decompression after filling the inner portion of the casting chamber with the filling medium.
 11. The duplex casting method according to claim 8, wherein a decompression rate of the casting chamber is regulated to regulate a speed at which the molten metal is injected into the mold.
 12. The duplex casting method according to claim 8, wherein the injection tube, the pressure regulation means, and the filling medium are reused. 